Alien planet 'could float on water'

A newly discovered alien planet is so large and light that it could float on water, posing new challenges to astronomers who are devising theories to explain how planets are born.

The gas-giant planet is about twice the size of Jupiter - the largest discovered so far - and its density as low as that of cork or balsawood, according to the study led by Georgi Mandushev of Lowell Observatory in Arizona to be published in the Astrophysical Journal.

The strange world is so hot, puffed up and diffuse that it is likely to have only a tenuous grasp on its outer atmosphere.

Astronomers predict it has a curved comet-like tail as it loses its grip.

The bloated planet lies in the constellation Hercules and is orbits the star catalogued as GSC02620-00648, which is about 440 parsecs (1,435 light-years) away from Earth.

The planet is about over four million miles away from its parent star - over ten times closer than Mercury is to our own Sun - and is heated by the intense starlight to about 2300 degrees Fahrenheit.

Because it makes a complete revolution around its local star every 3.55 days a year on this planet is shorter than a week on Earth.

The world is so big that it is hard to explain by current theory.

“We continue to be surprised by how relatively large these giant planets can be”, says Francis O’Donovan, a graduate student in astronomy at the California Institute of Technology, Pasadena.

“But if we can explain the sizes of these bloated planets in their harsh environments, it may help us better understand our own solar system’s planets and their formation.”

The alien world has been named TrES-4, to indicate that it is the fourth planet detected by the Trans-atlantic Exoplanet Survey network of three telescopes.